The explosive growth in wireless services over the past several years illustrates the huge and growing demand of the business community, consumers and the government for spectrum-based communications. Continuing advancements in technology, products and services are overwhelming the finite resources of the available communication spectrum. Industry has been forced to address dramatic changes, as it must adapt to accommodate the exponential demand on spectrum access, efficiency and reliability.
The Federal Communications Commission in the United States, and its counterparts around the world, allocate the radio spectrum across frequency channels of varying widths. One band can cover AM radio, another VH television, still others cell phones, citizen's-band radio, pagers, and so on. As more devices go wireless, they have to share a finite—and increasingly crowded—amount of radio spectrum. Although the radio spectrum is almost entirely occupied, not all devices use portions of the radio spectrum at the same time or location. At some locations or at some times of the day, a large percentage of the allocated spectrum may be sitting idle, even though it is officially accounted for.
Cognitive radio is a paradigm for wireless communication in which either a network or a wireless device uses spectrum that are licensed to other users or systems on a temporary secondary basis. This type of secondary access of unlicensed spectrum by a secondary system or cognitive device has to ensure minimum interference to the licensed users.
A primary system is a licensed (incumbent) user of a frequency spectrum. The primary system may be a television receiver that is within the service contour, licensed wireless microphones, or other systems governed by the FCC or other regulatory body. A secondary device could be a personal cell-phone, PDA or a communication network like a public safety network. There is an increased interest in deploying public safety devices in secondary spectrum to improve interoperability, range and data throughput and maintain mission criticality in case of emergencies and disaster management scenarios. Licensed Public Safety Networks typically utilize Frequency Domain Duplexing (FDD) as the FCC mandates paired frequencies in the Public Safety Licensed Spectrum. Conventional communication systems use Time Domain Duplexing (TDD) as means for communicating amongst radios and base stations. In order to increase spectrum availability in times of emergency or otherwise there is a need for FDD based public safety systems to utilize spectrum opportunistically on a secondary basis. Irrespective of the mode of operation, a secondary device must periodically sense for the presence of the primary and vacate that frequency or spectrum as soon as any primary activity is detected. In order to achieve this, the secondary systems utilize quiet periods during which all secondary activity is prohibited to allow the devices to sense for incumbent primary signals. Scheduling of these quiet periods is challenging and further, these periods of no activity could be longer and more frequent based on the system requirements and deployment areas thereby adversely affecting the overall network performance of the secondary system.
There is a need to protect the primary incumbent at all costs, but at the same time, manage the sensing activity in a way that minimizes the network down time.